April 14, 2016

Dealing with weather helm

A reader in Florida has just bought a new boat, the design of which shall be nameless. He is shocked by the amount of weather helm it possesses. “I remember that many years ago you wrote a column about how to tame weather helm,” he writes. “Can you please repeat it?”

With pleasure, sir. Here it is and I hope it helps you:

WEATHER HELM is not much discussed in polite sailing circles. In the same way that you don’t entertain party guests with tales of an ancestor hanged for treason, or a maiden aunt gone mad from syphilis, you don’t go around telling everybody your boat has weather helm, especially if you’re trying to sell it. Nevertheless, most boats have it, and if it’s excessive it’s a vexing thing to have.

Weather helm is the name we give to the tendency of a boat to round up into the wind. The term is actually incorrect, since weather helm is what the helmsman applies in an effort to counteract the tendency to round up, which is known as griping.

If your boat has a tiller, your arm can become mighty tired fighting weather helm. It’s an unrelenting tug that soon becomes much less than fun. Even if you have a wheel, and don’t have to counteract griping with sheer muscle power, excessive weather helm is a bad thing because putting the rudder over in an attempt to keep the boat going straight slows the boat down considerably and puts a heavy strain on the steering gear. In other words, like a leaky loo, weather helm is not a good thing to have.

So what causes it, and, more importantly, how do we cure it? Well, you might have to face the fact that it’s not always possible to cure it entirely, depending on the shape of your hull, the shape, size and position of your keel, and the position of your masts and sails.

What the designer seeks in the first place is a close balance between the center of effort (CE) of the sails and the center of lateral resistance (CLR) of the keel and the underwater hull and appendages.

Normally, the CE is a little forward of the CLR, because (just to make things more difficult) the CLR moves forward as the boat starts to move through the water. So it’s partly a guessing game with a new design. You may have seen boats like the Catalina 30 with little bowsprits added at a later stage. That’s an effort to move the CE forward, to counteract weather helm. But you have to be careful. Move it a little too far forward and you get lee helm, which is even worse than weather helm.

Some designs will always carry more weather helm than others. Hull types like the old IOR designs with a lot of beam carried a good way aft, and hard bilges, will quickly gripe in a puff. Boats with high-aspect-ratio rigs carry weather helm more quickly because the CE of the tall narrow sails is higher, so CE moves farther outboard over the water as the boat heels, thus pushing the boat from the side, and much farther out from the side, gaining leverage with every degree of heel.

Boats with blown-out, baggy sails suffer from weather helm because the CE moves aft. You can cure a bit of that, especially in rising winds, by tightening the halyards and flattening the sail any way you can, which will move the CE forward. The deepest bulge in a sail, the camber, always moves toward the edge under most strain. You can try that yourself with a handkerchief if you need convincing.

What other cures are there? Well, you could move the whole mast and rig forward. (Well, most of us couldn’t, actually, for obvious reasons.) You could rake the mast forward very slightly, or at least set it completely upright if it’s leaning aft. If you have a racing mast, a bendy mast, hauling on the backstay will induce an aft bend in the mast that will flatten the sail and reduce weather helm. In heavy winds you should set the mainsail traveler down to leeward as far as possible so that the sail spills wind and lies flatter. That helps quite a lot.

One thing often overlooked is that a large headsail can contribute to weather helm, too. Quite a lot of the area of your 150 percent genoa lies aft of the CLR, which is somewhere in the middle (in fore-and-aft terms) of your keel. You might as well be adding that extra genoa area to your mainsail. Change down to a smaller genoa or working jib, or roll it up to a similar size, and your CE will move forward.

And let’s not forget the best cure of all: reef the mainsail. Get rid of the sail area at the aft end of the boat that is constantly pushing the stern away from the wind and making the boat want to point up.

A little weather helm is a good thing. You don’t want it to disappear completely. You just need to be able to control it. Tank testing has shown that about 2 or 3 degrees of rudder from dead center helps lift a sailboat to windward. More than 4 degrees just acts as a brake to your progress.

In gusty weather, most of us will try to ride out the puffs by easing the mainsheet and putting the rudder over to leeward, but because excessive heeling is a major cause of weather helm it’s always wiser to reef down and keep the boat more upright if the wind is likely to continue at a greater strength.

Do what you can to lessen weather helm. It’s a good feeling to be in decent control of your boat in heavy wind. And I’ll tell you what — I won’t mention your weather helm to anyone if you don’t mention my maiden aunt.

Today’s Thought
It would have been as though he were in a boat of stone with masts of steel, sails of lead, ropes of iron, the devil at the helm, the wrath of God for a breeze, and hell for his destination.
—Emory A. Stones

“The doctor said I’d be on my feet in two weeks.”
“Was he right?”
“Yeah, I had to sell my car yesterday.”
(Drop by every Monday, Wednesday, Friday for another  Mainly about Boats column.)


Mike K said...

I have several books on yacht design by well known designers. When it comes to balance between the CE and the CLR they say there is no hard and fast rule. The CE almost always needs to be ahead of the CLR but by how much? Some designs are well balanced with a figure of 4-5%, others have 10%+ and need still more to reduce excess weather helm. It's all a bit trial and error, even in these days of CAD design. Designers also don't agree on whether the area of the rudder should be included or not when calculating CLR. According to my memory you seem to have covered all the bases John, except perhaps one. It is possible (though often difficult) to move the CLR aft without moving the keel. If the boat has a transom hung rudder, a skeg or enlarged skeg could be placed in front of it. Depending on the design of boat it may also be possible to install a movable (ie up and down) or permanent centreboard (or even two canted boards - one for each side) as far to the stern as possible. As weather helm is usually only an issue in stronger winds, these could be employed when needed on each tack. They will have the effect of moving the total CLR aft and increasing the lead of the CE. Vertical foils lose efficiency as the boat heels, so canted boards would gain in proportion against them, moving the CLR further aft as the boat heels which also helps. Toe (angle) them in to take further pressure off the rudder perhaps?

Alden Smith said...

Years ago I did a trip up to the Pacific Islands on a traditional long keeled yacht with hideous weather helm, it helped (as you comment on in your post) to put six rolls (roller reefing was very much in vogue then) in the main and that cured it until it blew even harder. I think the problem was that the immersed hull area wasn't particularly balanced - she had big shoulders just forward of the mast position. As the hull heeled the huge buoyancy in these 'shoulders' pushed the bow to windward.

It is interesting that yachts designed to the now discredited Meta-Centric meter rule popularized by the amateur English yacht designer T Harrison Butler all had almost perfectly balanced hulls without weather helm that steered themselves for long periods. The reason being that one side effect of the design rule was that it produced hulls where the underwater canoe body was symmetrical fore and aft. As the hulls designed to this rule heel they continue to project a relatively balanced immersed volume - there are no sudden changes of shape and pressure that alters the overall balance.

Mike K said...

In the 90's I had a 1/4 ton light displacement keelboat. It had a tall fractional rig with a reasonably high aspect ratio main, fin keel and transom hung rudder. It was very beamy, 24 foot long and a 10 foot beam. It was a delight to sail to windward, very well balanced and performed well to windward, especially in a blow. I sold it to a friend who 'turboed' it by adding a short prod for gennakers, jumpers to the rig for masthead kites and a larger main by lengthening the boom about a foot to 18 inches. He found that with the larger main it had developed weather helm. The stern was raked aft about 10 degrees with the rudder hung off this at the same angle. He notched the lower rudder pintle back into the stock and trimmed the corresponding leading edge of the stock back to the top pintle so that the rudder stood straighter with the leading edge of the blade now a few inches (sorry about all this old imperial stuff) in front of the pivot point, ie with some 'balance' to assist the helmsman. He claimed this made it much more pleasant and more or less cured the weather helm - although the CLR and CE were unchanged.